Heat in the Upper Regions of the Atmosphere. 105 



ascending ; and that of the latter by a curve intersecting the line 

 of abscissae at some distance from the former line, and perpetu- 

 ally approximating to it, as an hyperbola to its asymptote, as the 

 height increases. 



Such is the fact now ascertained ; and of the correctness of the 

 observations, which were numerous, there can be no doubt. In 

 other words, what is called the direct radiant heat of the sun 

 diminishes, and apparently without limit, by ascending to great 

 heights ; and if the experiment could be made, the shaded and 

 exposed thermometers outside the earth's atmosphere would show 

 the same very low reading. 



Some actinometrical observations were made which, as far as 

 they went, confirmed, the conclusion above stated. More of 

 them, however, troublesome as they must be in a balloon, are 

 much needed. 



Accepting, then, the fact, we may indicate a few of the infer- 

 ences which may be drawn from it. The very small power of 

 the sun to melt snow and ice at high altitudes among moun- 

 tains is more satisfactorily accounted for than it has previously 

 been. All those views of the state of the surface of the moon 

 which represent it as alternately exposed to fiery heat and 

 intense cold will disappear. The temperature on the dark and 

 bright halves of the half moon is the same, if there is no atmo- 

 sphere. We learn one more marvellous function of our atmo- 

 sphere before unsuspected. We knew that it and the aqueous 

 vapour it bears diffused through it save us from loss of heat. 

 We did not know that but for it, or some part of it, we should 

 have no heat to lose. Hence, too, we learn that conclusions as 

 to the temperatures of other planets are still more open to un- 

 certainty, as being in this way also influenced by their atmo- 

 spheres. 



Two theories respecting this fact suggest themselves. First, 

 that heat-waves do speed through space and our atmosphere, 

 and both strike on the surface of solid bodies exposed to them, 

 and communicate a motion to adjacent molecules not so exposed 

 directly, which in their turn pass on the motion to others. The 

 latter part of this motion is known as the temperature of the 

 shaded bulb, the former as that of the exposed bulb. But the 

 difference between the two readings indicates only the excess of 

 the motion received over that which it sends back ; and it is con- 

 ceivable that the density of the surrounding gas may greatly 

 affect the amount so sent back, so that the excess may diminish 

 with the density of the gas, and therefore with the altitude. The 

 density of the surrounding gas may also greatly affect the ab- 

 sorbing powers of the surface ; and the blackened bulb, which 



